1. Field of the Invention
This invention relates to a metal gasket for obtaining high air-tightness resulting from pressing on seal face of a sealing portion on which the metal gasket is placed.
2. Description of Related Art
In the manufacturing process of semiconductor, processes for thermodiffusing ultra-toxic gases such as silane, arsine, phosphine, and diborane on a silicon wafer base as impurity source have been employed and spontaneous-ignitable gases have been used.
Such gases used in the manufacturing process of semiconductor involves the risk of direct physical injury, for example, the lethal dose of arsine is 0.5 ppm and a little leakage of the gas causes physical injury, conventionally piping for such gas has been jointed by welding or sealing using a gasket for preventing leakage. Because such gases are usually toxic and also corrosive, a gasket consisting of rubber or resin can not be used and a metal gasket has been used for gasket sealing.
For equipments for manufacturing of semiconductors which involves handling of such hazardous gas, various metal gaskets has been proposed and used for high air-tightness. The conventional metal gasket will be described referring examples herein under.
FIG. 6 is a perspective view for illustrating the first conventional example of a metal gasket. The metal gasket 51 comprises a metal sheath with two layered structure having inside-sheath 53 and outside-sheath 54 with a cross-section in the form of C-shape provided adjacent each other in the axial direction and a metal core 52 comprising an elastic metal coil spring in the form of ring shape provided on the inside of the inside-sheath 53.
Otherwise, the second example of a conventional metal gasket disclosed in Japanese Unexamined Patent Publication No. Hei 2-138575 is shown in FIG. 7. The metal gasket 61 comprises a metal core 62 comprising a ring hollow elastic metal coil spring with an elliptic cross-section and an outside single-layered metal sheath 63 with a C-shaped cross-section surrounding the peripheral surface of the metal core 62 continuous in the axial direction.
When a gasket 61 having such a structure receives a compression force exceeding a prescribed value from the bracket 60 in the minor axis direction, the top and bottom sides deflect at the central portions to form arched surfaces 64 and 64 respectively as shown in FIG. 8, and two circles 65 and 66 between the arched surfaces 64 and 64 are served to seal the top and bottom faces. High sealing effect is exhibited by doubled sealing lines.
However, recently it is desirable to use austenite stainless steel, particularly SUS316 (referred to as merely SUS hereinafter), for it is excellent in corrosion resistance as sheath material of metal gaskets in stead of nickel for it can generate poison through reaction with a gas, however, SUS is disadvantageous in that the stability of sealing is poor.
The reason is described herein under. SUS316 has the hardness of as high as about Hv 320 in comparison with the hardness of nickel on the outside sheath surface of Hv 180. on the other hand, a flange on which the metal gasket is to be mounted has the hardness of about Hv 300 because the flange is formed of SUS316 or VIM-VAR material subjected to burnishing surface processing.
Accordingly, when a sheath consisting of nickel is used and there is hardness difference between a flange and the sheath, the sealing portion is strained on the sealing face of the bracket to make sure the air-tightness.
However, when there is no hardness difference between a flange and the sheath because high hardness material such as SUS is used for the sheath, the metal gasket is not strained to fit evenly on the sealing face, the stability of sealing is poor, and the poor sealing is a problem.
When there is hardness difference between a flange and the sheath because nickel is used for the sheath, the sheath is pressed on the sealing face with high pressure to cause strain, the material of the sheath is forced to penetrate into the sealing face of the flange to enhance the air-tightness.
However, when there is no hardness difference between the sheath of high hardness material such as SUS and a flange, because both the gasket of the first example with a circular cross-section and of the second example which forms double sealing lines with elliptic cross-section contact with a bracket on the curved surface to render increased contact surface area relatively and such contact between surfaces involves low areal pressure, the material of the sheath is not forced to penetrate to the sealing face not to enhance the air-tightness, it is disadvantageous in that the stability of sealing is poor.
Therefore, a very high pressing force is required to enhance the sealing stability.
It is the object of the present invention to provide a metal gasket excellent in sealing stability without such problem and capable of effectively preventing fluid from leakage with a reduced pressing force.